Why Do We Have Bums?

When separates us from the animals? Is it the burden of consciousness, the terror of knowing that one day we will die, along with everyone we’ve ever loved? Or is it our big weird arses?

The answer is both, probably. But while whole careers have been spent tracing the origins of human self-awareness, the lowly bum cheek has to this point not warranted even a short monograph. These fleshy, sometimes hairy, alternately hilarious, erotic and mundanely functional appendages — where did they come from? And what purpose were they shaped to serve?

For this week’s Giz Asks, we reached out to a number of experts who, in the course of their careers, have for one reason or another come to intimately know the evolutionary history and present-day biology of the human arsecheek. It’s a story that starts 350 million years ago, with massive primordial tails, and—winding its way through the millennia — ends on those time-whittled marvels of ergonomic flesh-and-muscle you’re probably sitting on right now.

Jason Bourke

Paleontologist and Assistant Professor of Anatomy, New York Institute of Technology College of Osteopathic Medicine at Arkansas State

The structures we refer to as butt cheeks in humans are comprised of the gluteal muscles: gluteus minimus, gluteus medius, and gluteus maximus. Of the three gluteal muscles, gluteus maximus is responsible for the signature shape of the human derriere. It originates from a line that runs from our upper ilium (the pelvic bone that people often call their hips) down towards our coccyx (tailbone). The muscle attaches close to the top of our femur (thigh bone). The gluteus maximus functions as our major leg extensor and is the main driver of our legs when running, climbing stairs, getting up from a seated position etc. It’s the powerhouse muscle we call upon whenever we need to perform major postural changes, or when we need to move our legs fast. That’s why sprinters, and weightlifters that do heavy-weighted squats tend to have very round and firm buttocks (i.e., “squat butt”).

Because the muscle originates and inserts across a fairly small distance, it needs to produce a lot of force to get our legs to move. This makes the muscle very large, but since there is not a lot of space for the muscle to sit, the muscle fibres expand outwards and—thanks to gravity—downwards resulting in our hallmark hind ends.

The shape of our rear end is practically unique to our species. As we transitioned from quadrupedal apes to bipedal ones, our pelvis underwent radical changes to handle the weight of our entire torso resting on top of it. This required substantial reorientation of many of our hip muscles and it put our major leg extensor (gluteus maximus) in this weird position where it seems to almost hang off of our pelvis. There are a few other species of mammals that have what we might term “butt cheeks”. Namely horses, which show substantial developments of their rear ends as well. As with humans, well-developed gluteal muscles are responsible for the roundness of horse butts. However, unlike humans, horses achieved this shape via expansion of a different gluteal muscle, their gluteus medius. In fact, a large gluteus medius is pretty standard for most mammals. Humans are unique in expanding our gluteus maximus instead, which is no doubt a response to the unique physical demands of our strange way of walking.

Francois Therrien

Curator of Dinosaur Palaeoecology, Royal Tyrell Museum

The first four-legged animals to walk on land 350 million years ago had large tails. One of their tail muscles, the caudofemoralis, which links the base of the tail to the thigh bone (femur) and was used for swimming in their aquatic ancestors, could now be used to pull the hind limb backwards, which helped the animals to propel themselves forward. Through evolution, this muscle was retained in later animals and is still being used by a vast group of animals alive today, such as amphibians and reptiles. These animals often have long and massive tails, because they rely heavily on the caudofemoralis for walking.

However, things changed 300 million years ago in a group of animals called synapsids, the ancestral lineage of mammals. In these animals, the importance of the tail diminished and a different set of muscles was recruited to help with locomotion: the gluteal muscles. These muscles link the hip to the thigh bone without attachment to the tail. Over the course of millions of years, the tail of synapsids became smaller and smaller through evolution as the caudofemoralis muscle was no longer involved in walking (by the time the first mammals evolved over 200 million years ago, the tail was very small in comparison to their body and was no longer used for locomoion) while the gluteal muscles increased in size and formed the rump.

With the evolution of bipedality (two-legged stance) in the first human ancestors 6 million years ago, these primates stopped using their forelimbs for locomotion and had to rely exclusively on the hindlimbs to walk upright. This transition meant that all the power needed for propulsion (i.e., walking) had to be generated exclusively by the gluteal muscles, which as a result had to further increase in size and became more prominent. Add to this a few fatty pads strategically located for “cushioning,” so the gluteal muscles don’t get damaged by rubbing against the hip bones while walking or sitting, and you have the evolution of formal “butt cheeks.”

So this is why you don’t find butt cheeks in dinosaurs or in non-human mammals: the former still used a caudofemoralis muscle attached to a tail for walking, whereas the latter walk quadrupedally.

Kirsten Brown

The first thing to understand is that humans are the only habitually bipedal primates. Gorillas and chimpanzees can walk bipedally for short periods of time, but they’re not doing it to the extent that humans are. Because of these (and other) behavioural differences, we also see differences in our anatomy. In humans, the pelvis is what we would called flared. The upper portions of the pelvis, known as the ilia, are positioned to the side of the body. In apes, these bones are positioned toward the back.

Without getting into the many theories of how bipedality evolved, it’s important to keep in mind that ultimately different bone orientations = different muscle function = differences in locomotion.

For humans, locomotion means walking on two legs, with an alternation between two-leg support versus single-leg support. Most of the time you walk ( 75%), you’re supporting your body on a single limb. In order to do this you need to recruit many muscles for stability and support, including the gluteal muscles, which help to define the shape of the buttocks.

The largest of the gluteal muscles is gluteus maximus, which makes up a large portion of this region and spans from the pelvis (i.e., hip bone) to the femur (i.e., upper thigh bone). Its size is not only manifested in how wide its attachments are, but also in how thick it is, and it plays a chief role in decelerating the lower limb and stabilising your trunk. You don’t want your trunk and upper body moving faster than the lower limbs will allow or you’ll fall over.

The gluteal abductors—another part of your butt’s anatomy—function to prevent what is known as pelvic tilt. If you stand on one leg, your pelvis is approximately level. The gluteal muscles contract on your standing leg to prevent the pelvis from falling to the opposite side. This may seem inconsequential, but for people with injuries to these muscles it’s not. In individuals with Trendelenberg gait, they have an injury to the muscles or more commonly to the nerve supplying these muscles. Consequently, they’re not capable of maintaining a stable pelvis while standing on one leg. Therefore they automatically will shift their body weight and trunk excessively to prevent falling over.

Susan Larson

Professor and Chair, Anatomical Sciences, Stony Brook University

The contour of the human buttock is mainly due to a large muscle aptly named gluteus maximus (gluteus is from Greek gloutos meaning buttock). It is the largest muscle in human anatomy, and its size and configuration is distinctive in humans compared to our primate relatives. It acts to extend the thigh (i.e, move it backwards) and its large size is generally related to the fact that humans stand with a thigh that is already more extended than you see in other primates, and yet we are able to extend it even further, that is, past vertical. The force it produces is very important during running and climbing stairs as well as standing from a sitting position.

That being said, there is more fat in the gluteal region than you might expect, which contributes to the contour of the buttock. It has been suggested that this may be due to sexual selection, that is, we find a rounded gluteal region “sexy.” This is of course speculation so it is hard to prove one way or another.

Jason M. Organ

Assistant Professor of Anatomy and Cell Biology

The lesser gluteal muscles of humans no longer move the thigh backward at the hip joint like they do in other Great Apes. Instead, they move the thigh outward from the side of the body. This new action of the lesser gluteals is critical to avoid falling over during walking on two legs, but it reduces the number of muscles that can move the thigh backward, which is an equally important movement when trying to move the body forward in walking. This is why the gluteus maximus is so large: it has to be in order to make up for the loss of two other muscles that had the same action! Of course, none of this explanation takes into account the possibility that our ancestors liked big butts (and they could not lie). Perhaps the size of the gluteus maximus was also influenced by what our evolutionary ancestors found attractive in their potential mates.

Daniel E. Lieberman

Edwin M Lerner Professor of Biological Sciences and Chair, Department of Human Evolutionary Biology at Harvard University, and the author of The Story of the Human Body

If you mean the muscle, the answer is running. The muscle making up the butt is the gluteus maximus (yes, the largest in the human body) and its upper (cranial) portion is vastly expanded in humans compared to ape. We barely use it when we walk, but when we run its plays a key role in controlling the trunk, extending the leg behind you, and slowing the leg you are swinging in front of you. Interestingly we can see from the fossil record this muscle expansion occurred around 2 million years ago with the evolution of H. erectus and the origins of endurance running.

The other key component of our butts is fat, and even the skinniest humans are fat compared to other mammals because we need more fat for our expensive reproductive and energetic strategy.

Steven Churchill

Professor, Evolutionary Anthropology, Duke University

As our ancestors evolved to become proficient bipeds (that is, proficient at walking on two legs), the pelvis became shorter and wider so that the two small gluteal muscles (“butt muscles,” so to speak), gluteus medius and gluteus minimus, migrated around to the sides to help us stabilize the pelvis when we balance on one leg as we swing the other leg forward. In apes, these muscles serve a propulsive role, but in us they perform this balancing function. In turn, we have enlarged the large gluteal muscle, gluteus maximus, as a propulsive muscle that we use when running or when climbing hills or stairs. These enlarged gluteus maximus muscles give humans our distinctive butt cheeks.